Fuel processing system for desulfurization of fuel for a fuel cell power plant

ABSTRACT

A fuel processing system ( 14 ) removes sulfur from fuel cell fuels such as ethanol and methanol. The system ( 14 ) directs the fuel through a fuel vaporizer ( 26 ), reformer ( 32 ), carbon monoxide conversion station ( 44,48 ) and through a sulfur scrubber station ( 52 ). The fuel is then directed into an anode flow field ( 16 ) of a fuel cell ( 12 ) of a fuel cell the power plant ( 10 ). By converting the carbon monoxide prior to removing sulfur from the fuel, no carbon monoxide is available to form gaseous carbonyl sulfide within the sulfur scrubber station ( 52 ). Because no carbonyl sulfide is formed, sulfur adsorption material within the scrubber station ( 52 ) may adsorb elemental sulfur from the fuel equal to between about fifteen percent and sixty percent of a weight of the sulfur adsorption material so that regeneration of the sulfur adsorption material is not necessary.

TECHNICAL FIELD

The present disclosure relates to fuel cells that are suited for usagein transportation vehicles, portable power plants, or as stationarypower plants, and the disclosure especially relates to a system andmethod of desulfurization of fuel for a fuel cell power plant.

BACKGROUND ART

Fuel cells are well known and are commonly used to produce electricalcurrent from hydrogen containing reducing fluid fuel and oxygencontaining oxidant reactant streams to power electrical apparatus suchas transportation vehicles. As is well known in the art, a plurality offuel cells are typically stacked together to form a fuel cell stackassembly which is combined with controllers and other components to forma fuel cell power plant. In fuel cells of the prior art, it is wellknown that fuel is often processed through a reformer and the resultingreformate fuel flows from the reformer through one or more fueltreatment stations into and through anode flow fields of the fuel cellsof the stack. An oxygen rich reactant simultaneously flows through acathode flow field of the fuel cell to produce electricity.Unfortunately, known fuels for fuel cells, such as reformate fuels fromreformers, frequently contain contaminants especially sulfur, which isdetrimental to the performance of the fuel cell.

SUMMARY

It is increasingly common to consider renewable energy sources such asethanol or methanol as a fuel source for a reformer of a fuel cell powerplant. Unfortunately, there are no known methods of efficientlydesulfurizing ethanol or methanol. Where methanol has been utilized forexperimental fuel cell power plants powering urban buses, only a veryexpensive, ultra-pure grade of methanol may be used to minimize sulfurcontamination of the fuel cells. Similarly, the renewable fuel ethanolalso contains small amounts of sulfur (e.g., 1-2 parts per million(“PPM”)). Further complicating use of such fuels is a requirement thatultra-pure, or extremely low sulfur content fuels, must be transportedto fuel cell power plants in dedicated fuel delivery systems to avoidsulfur contamination from high-sulfur content fuels transported innon-dedicated fuel delivery systems.

One effort at desulfurization of fuel for a fuel cell power plant isdisclosed in U.S. Pat. No. 6,610,265 that issued on Aug. 26, 2003 toSzydlowski et al., which patent is owned by owner of all rights in thepresent invention. Szydlowski et al. shows a complex system and methodthat includes parallel desulfurization beds through which the fuelflows. While one desulfurization bed is being utilized to desulfurize areformate fuel, the other desulfurization bed is being regenerated by agas stream containing carbon monoxide.

In the system and method disclosed in Szydlowski et al. a reformate fuelflows first through a sulfur scrubber station that includes the twobeds, and then through an ammonia removal bed and then through a carbonmonoxide reduction station to minimize the amount of carbon monoxidewithin the fuel. The fuel is then directed into an anode flow field of afuel cell. The sulfur scrubber station or bed converts hydrogen sulfidein the gaseous fuel stream to elemental sulfur through the Clausreaction with an addition of a small amount of atmospheric oxygen. Theelemental sulfur then precipitates out of the gaseous stream onto asurface of a sulfur adsorption material in the scrubber. Once sulfuraccumulates on the adsorption material surfaces the carbon monoxide inthe fuel stream begins to react with the sulfur to form carbonyl sulfide(COS) which is carried to the anode, poisoning the anode. As a result,the fuel stream must be switched to a parallel bed to avoidcontamination of catalysts of the fuel cell by the COS flowing with thefuel. The sulfur scrubber bed with the accumulated elemental sulfur canbe regenerated by directing a gaseous stream containing at least onepercent by volume carbon monoxide. The carbon monoxide convertselemental sulfur to gaseous carbonyl sulfide (COS), which is thendirected to flow out of the bed.

While the Szydlowski et al. desulfurization system and method achievesacceptable results by producing exit sulfur levels of less than tenparts per billion, the system is very complex and therefore involvessubstantial cost in manufacture, assembly and operation, and necessarilyrequires a large volume of the power plant to house all if itscomponents. This is especially important for any types of mobile fuelcell power plants where space and weight are critical to a successfuldesign. Consequently there is a need for a system and method ofdesulfurizing fuel for a fuel cell power plant that minimizesmanufacture, assembly and operating costs, and that requiressignificantly less volume of the power plant.

The disclosure includes a fuel processing system for desulfurization ofa hydrocarbon fuel for a fuel cell power plant. The power plant has atleast one fuel cell having an anode flow field and a cathode flow fielddisposed on opposed sides of an electrolyte. A supply of a hydrocarbonbased hydrogen rich fuel is directed from a fuel source through a fuelinlet line into the anode flow field. The fuel processing systemincludes a fuel vaporizer, a reformer, a water gas shift reactor device,a preferential selective oxidizer device (the water gas shift reactordevice and preferential selective oxidizer device may be collectivelyreferred to as a carbon monoxide conversion station), all of which aresecured in fluid communication through the fuel inlet line with the fuelsource. The fuel processing system also includes a sulfur scrubberstation that is secured in fluid communication with and downstream fromthe carbon monoxide conversion station for removing sulfur from the fuelpassing through the sulfur scrubber station. The sulfur scrubber stationincludes an air inlet for selectively feeding air into the scrubberstation.

The fuel processing system feeds the anode flow field of the fuel cellwhich is secured in fluid communication, through another extension ofthe fuel inlet line, with and downstream from the sulfur scrubberstation so that the fuel flows from the sulfur scrubber station throughthe anode flow field.

By configuring the sulfur scrubber station to be downstream from thecarbon monoxide conversion station the fuel entering the sulfur scrubberstation has a minimal amount of carbon monoxide typically less than fiveparts per million (PPM). Therefore, as elemental sulfur is precipitatedonto surfaces of sulfur adsorption material within the sulfur scrubberstation, not enough carbon monoxide is available to form gaseouscarbonyl sulfide. Any gaseous carbonyl sulfide would leave the scrubberstation within the fuel stream and pass into the anode flow field tocontaminate the fuel cell. Because no carbonyl sulfide is formed, thesulfur adsorption material within the scrubber station may adsorb a verysubstantial amount of elemental sulfur. For example, a preferredmaterial in the sulfur scrubber station may be formed from apotassium-promoted activated carbon or other known sulfur selectivecarbons so that the sulfur adsorption material may adsorb as muchelemental sulfur as between about 15 percent and about 60 percent of theweight of the material. In contrast, sulfur scrubbers of knowndesulfurizing systems have only been able to adsorb between about 0.5 to1.0 percent sulfur.

Because such an enormous amount of sulfur may be removed from a fuelpassing into the fuel cell, it is not necessary to regenerate the sulfurscrubber station. Instead, depending upon operational parameters of aparticular fuel cell power plant, a sulfur adsorption material bed of asulfur scrubber station of the present invention may simply be removedand replaced at predetermined intervals, if necessary. In a preferredembodiment of the system, the fuel is produced by a reformer andpreferred fuels supplied to the reformer include methanol and ethanol.

Accordingly, it is a general purpose of the present disclosure toprovide a fuel processing system and method for desulfurization of fuelfor a fuel cell power plant that overcomes deficiencies of the priorart.

It is a more specific purpose to provide a fuel processing system andmethod for desulfurization of fuel for a fuel cell power plant thatminimizes manufacturing, assembly and operating costs and displacementvolume within the power plant.

These and other purposes and advantages of the present fuel processingsystem and method for desulfurization of fuel for a fuel cell powerplant will become more readily apparent when the following descriptionis read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a simplified schematic representation of a fuel cell powerplant including a fuel processing system constructed in accordance withthe present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in detail, a fuel processing system fordesulfurization of a hydrocarbon fuel for a fuel cell within a fuel cellpower plant is shown in FIG. 1. The fuel cell power plant is generallydesignated by the reference numeral 10. The power plant 10 includes atleast one fuel cell 12 as part of the fuel cell power plant 10, and thefuel cell 12 includes an anode flow field 16 and a cathode flow field 18disposed on opposed sides of an electrolyte 20. The fuel processingsystem is generally designated by the reference numeral 14 in FIG. 1,and is described in more detail below as a system of the fuel cell powerplant 10.

Within the power plant 10 a hydrocarbon based liquid fuel is stored in afuel source 22 and may be selectively directed from the source 22through a fuel inlet line 24 into and through the fuel processing system14. The system 14 includes a fuel vaporizer 26 wherein a supply of steam28 passing into the fuel vaporizer 26 vaporizes the fuel. The gaseousfuel then flows through a first extension 30 of the fuel inlet line 24into a reformer 32 of the fuel processing system 14. The reformer 32 mayreceive a supply of air 34 and potentially more steam. The reformer 32may be an auto-thermal reformer, a partial oxidation reformer, or anyreformer means known in the art for transforming a hydrocarbon basedfuel into a hydrogen gas (H₂) commonly called a reformate fuel stream.In addition to hydrogen, the reformation process also converts sulfurwithin the fuel stream into hydrogen sulfide (H₂S). The reformate streammay also include other gases, such as carbon monoxide, carbon dioxide,water, nitrogen, methane, ammonia and trace compounds. The reformatefuel stream is then directed to flow from the reformer 32 by a secondextension 36 of the fuel inlet line 24 through a cooler/heat exchangeror exchangers 38 that receives a supply of coolant from a coolant inletline 40 to control a temperature of the fuel stream within a desiredrange.

The fuel then moves from a third extension of the fuel inlet line 24through a plurality of treatment stations. A first station mayoptionally be an ammonia removal station 43, which is not consideredpart of the fuel processing system 14 of the present invention. The fuelstream is then directed by an additional inlet line extension 42 througha carbon monoxide conversion station 45 which is part of the fuelprocessing system 14. The carbon monoxide conversion station 45 mayinclude a water gas shift converter device 44, to lower carbon monoxideto about 0.5 to 1.0 percent, followed through inlet line extension 46 bya preferential selective oxidizer device 48, which includes air bleedline 47, for reducing the carbon monoxide level in the fuel stream toabout five parts per million (PPM). Next, the fuel is directed byanother extension 50 of the fuel inlet line 24 into and through a sulfurscrubber station 52 of the fuel processing system 14. The sulfurscrubber station 52 removes sulfur, typically in the form of hydrogensulfide, from the fuel stream. The sulfur scrubber station 52 may be anysulfur scrubber station device or means for removing sulfur known in theart. Preferably the sulfur scrubber station or device 52 includes a bedcontaining potassium-promoted activated carbon or other known materialseffective to promote the Claus reaction, such as Group 1 metals on alarge surface area support material. The support materials and the bedwithin the sulfur scrubber station or device 52 are virtually the sameas those described in the aforesaid U.S. Pat. No. 6,610,265 toSzydlowski et al. The materials are the U.S. Filter/Westates UOCH-KPcarbon and sulfur selective carbon. Temperatures within the scrubberstation 52 are maintained a few degrees above a dew point of the gasstream, about 170 degrees Fahrenheit or slightly higher, and about 0.5percent oxygen is added to the fuel stream. The gaseous fuel passes overand through the sulfur scrubber station or device 52 and any of theaforesaid catalysts and, with the addition of a small amount of airthrough an air inlet 54. The Claus reaction causes gaseous hydrogensulfide to react with oxygen and form elemental sulfur and water. Theelemental sulfur is adsorbed on surfaces of carbon within the sulfurscrubber station or device 52. The fuel stream is then directed througha sixth extension 56 of the fuel inlet line 24 into the anode flow field16 of the fuel cell 12. Simultaneously, a flow of an oxygen richreactant stream, such as the air, is directed through an oxidant inletline 58 through the cathode flow field 18 of the fuel cell 12 so as toproduce electricity. An anode exhaust 60 and a cathode exhaust 62 aresecured in fluid communication with the anode and cathode flow fields16, 18 to direct the fuel and oxidant out of the fuel cell 12.

In a preferred embodiment, the fuel processing system 14, which includesthe carbon monoxide conversion station 45 and the sulfur scrubberstation 52, may be fed by the reformate gas stream from the reformerwhich is secured in fluid communication through the fuel inlet line 24with the fuel source 22 so that the fuel directed into the fuel inletline 24 from the reformer 32 is a reformate fuel stream. The carbonmonoxide conversion station 45 is secured downstream from and in fluidcommunication with the reformer 32 and the carbon monoxide conversionstation 45 is configured to direct flow of the fuel through the station45 to convert carbon monoxide in the fuel to benign products, primarilycarbon dioxide. The sulfur scrubber station 52 is secured in fluidcommunication with and downstream from the carbon monoxide conversionstation 45. The sulfur scrubber station 52 also has an air inlet 54 andthe sulfur scrubber station 52 is configured to direct the fuel throughthe station to remove sulfur from the fuel. The sulfur scrubber stationis also configured to direct the fuel into the anode flow field 16 ofthe fuel cell 12 that is secured in fluid communication through the fuelinlet line 24 with and downstream from the sulfur scrubber station 52.Therefore, the fuel flows from the fuel source 22 through the fuel inletline 24 to and through the reformer 32, to and through the carbonmonoxide conversion station 45, to and through the sulfur scrubberstation 52, and to and through the anode flow field 16 of the fuel cell12.

The reformer means 32 may also be a sulfur tolerant reformer. The carbonmonoxide conversion station 45 preferably leaves about five PPM or lesscarbon monoxide within the reformate fuel stream. Removal of virtuallyall of the carbon monoxide allows the sulfur scrubber station 52 to holdbetween about 15 percent and about 60 percent of the weight of thesulfur adsorption material within the sulfur scrubber station 52.Additionally the fuel stream entering the sulfur scrubber station 52 iscontrolled by the cooler 38, or by any other temperature control means(not shown) known in the art for controlling a temperature of a fuelstream within a fuel cell power plant, so that the temperature of thefuel stream within the sulfur scrubber station 52 is above the dew pointof the fuel stream, and is also below a temperature at which hydrogengas and oxygen gas within the fuel stream would ignite.

The disclosure also includes a method of desulfurization of fuel for afuel cell power plant 10 by first reforming the fuel to produce ahydrogen rich reformate fuel stream containing sulfur compounds and toturn sulfur within the fuel into hydrogen sulfide; then, convertingcarbon monoxide from the reformate fuel stream passing through the fuelinlet line 24 from a fuel source 22 to an anode flow field 16 of a fuelcell 12 by flowing the reformed fuel through the carbon monoxideconversion station 45; then, removing sulfur from the fuel by passingthe fuel through the sulfur scrubber station 52 while simultaneouslyflowing air through an air inlet 54 through the scrubber station 52;and, then directing flow of the fuel from the sulfur scrubber station 52into and through the anode flow field 16 of the fuel cell 12.

In an additional preferred embodiment, preferred fuels include ethanoland methanol. In particular, ethanol is an increasingly popular andrenewable fuel. Unfortunately, efficient removal of sulfur from ethanolat levels necessary for efficient operation of a fuel cell power planthas so far proven difficult and impractical. By the present disclosure,however, it is now possible to utilize either ethanol or methanol asfuels for a fuel cell power plant wherein the fuels are reformed by thereformer 32 into a hydrogen rich reactant stream containing sulfur andrelated contaminants described above. Through use of the present fuelprocessing system 14 with such fuels, it would no longer be necessary toutilize special dedicated fuel delivery systems to transport fuel from apoint of origin to the fuel cell power plant 10 as is presently done toavoid contamination of ultra-pure, low sulfur content fuels.Additionally, the present fuel processing system 14 and method alsoefficiently removes sulfur from other common hydrogen rich hydrocarbonfuels such as gasoline, diesel fuel, natural gas, liquid petroleum gas(LPG_) etc.

In the present fuel processing system 14 and method of desulfurizationof fuel for the fuel cell 12, within the sulfur scrubber station 52 anysulfur within the reactant fuel stream in the form of hydrogen sulfideis reacted with oxygen to form elemental sulfur and water. The elementalsulfur is then adsorbed in pores of sulfur adsorption material withinthe scrubber station 52. However, because any carbon monoxide has beenremoved from the reactant fuel stream prior to entry of the fuel intothe sulfur scrubber station 52, virtually no gaseous carbonyl sulfide(COS) is formed from the elemental sulfur or other forms of sulfurwithin the sulfur scrubber station 52. This is important because COS hasa negative effect on the performance of the fuel cell 12 and thereforethe presence of COS must be avoided within the reformate fuel streamentering the fuel cell 12. The presence of carbon monoxide in thedesulfurization station 52 also limits an ability of the sulfur scrubberstation 52 to hold a significant weight percent of sulfur. If the fuelprocessing system 14 did not remove virtually all of the carbonmonoxide, then much larger, or multiple desulfurizing beds would berequired.

It is considered that part of the present disclosure is the discovery bythe inventors herein that the formation of carbonyl sulfide within priorart desulfurization systems severely limited a holding capacity of thesulfur scrubber beds of prior art desulfurization systems or fuelprocessing systems. Instead of resolving that problem by complicated,parallel, on-off cycling sulfur scrubber beds, the present inventionconverts carbon monoxide to other benign species prior to removingsulfur so that carbonyl sulfide cannot be formed within the fuel. As aresult, preferred sulfur adsorption materials within the sulfur scrubberstation 52 may hold significantly more sulfur. A preferred sulfuradsorption material in the sulfur scrubber station 52 may be formed froma potassium-promoted activated carbon or other known sulfur selectivecarbons. Such sulfur adsorption materials as described above may adsorbas much elemental sulfur as between about 15 percent and about 60percent of the weight of the sulfur adsorption material. (For purposesherein the word “about” is to mean plus or minus 10 percent.) Incontrast, sulfur scrubbers of known fuel processing or desulfurizingsystems have only been able to adsorb between about 0.5 to 1.0 percent.

While the present disclosure has been presented with respect to thedescribed and illustrated fuel processing system 14 for desulfurizationof fuel for a fuel cell power plant 10, it is to be understood that thedisclosure is not to be limited to those alternatives and describedembodiments. For example, the fuel processing system 14 may be utilizedwith any fuel cells including phosphoric acid fuel cells, protonexchange membrane fuel cells, etc. Accordingly, reference should be madeprimarily to the following claims rather than the forgoing descriptionto determine the scope of the disclosure.

1. A fuel processing system (14) for a fuel cell power plant (10)operating on a sulfur containing fuel, the power plant (10) having atleast one fuel cell (12) including an anode flow field (16) and acathode flow field (18) disposed on opposed sides of an electrolyte(20), the fuel processing system (14) comprising: a. a fuel vaporizer(26) secured in fluid communication through a fuel inlet line (24) witha fuel source (22); b. a sulfur tolerant reformer (32) secured in fluidcommunication through an extension of the fuel inlet line (24) with thefuel vaporizer (26); c. a carbon monoxide conversion station (45)secured in fluid communication through an extension of the fuel inletline (24) with the reformer (32); d. a sulfur scrubber station (52)secured in fluid communication, through an extension of the fuel inletline (24), with and downstream from the carbon monoxide conversionstation (45) for removing sulfur from the fuel passing through thesulfur scrubber station (52), the sulfur scrubber station (52) includingan air inlet (54) for selectively permitting air into the scrubberstation (52); and, e. the anode flow field (16) of the fuel cell (12)being secured in fluid communication with and downstream from the sulfurscrubber station (52) through an additional extension of the fuel inletline (24), so that fuel flows from the fuel source (22) through the fuelinlet line (24) sequentially to and through the fuel vaporizer (26), thereformer (32), the carbon monoxide conversion station (45), the sulfurscrubber station (52), and to and through the anode flow field (16) ofthe fuel cell (12).
 2. The fuel processing system (14) of claim 1,wherein the carbon monoxide conversion station (45) comprises a watergas shift reactor device (44) in fluid communication with and upstreamfrom a preferential selective oxidizer device (48) secured in fluidcommunication with the fuel inlet line (24).
 3. The fuel processingsystem (14) of claim 1, wherein the fuel is selected from the groupconsisting of ethanol, methanol, gasoline, diesel fuel, natural gas,liquid petroleum gas (LPG) and combinations thereof.
 4. The fuelprocessing system (14) of claim 1, wherein the sulfur scrubber station(52) includes sulfur adsorption material selected from the groupconsisting of potassium-promoted activated carbon, Group 1 metals on asupport material, and other materials known to effect the Clausreaction.
 5. A method of desulfurizing a hydrocarbon fuel for a fuelcell power plant (10), the power plant (10) having at least one fuelcell (12) including an anode flow field (16) and a cathode flow field(18) disposed on opposed sides of an electrolyte (20), the methodcomprising: a. vaporizing the hydrocarbon fuel within a fuel vaporizer(26) secured in fluid communication through a fuel inlet line (24) witha fuel source (22); b. supplying the vaporized fuel to a sulfur tolerantreformer (32); c. reforming the vaporized fuel within the reformer (32)into a hydrogen rich gas stream containing hydrogen sulfide gas andcarbon monoxide; d. supplying the hydrogen rich gas stream containingthe hydrogen sulfide gas and carbon monoxide to a carbon monoxideconversion station (45) and reducing the carbon monoxide content in thegas stream to less than five parts per million; and, e. supplying thehydrogen rich, carbon monoxide reduced gas stream to a sulfur scrubberstation (52) while also injecting air into the sulfur scrubber station(52) thereby converting the hydrogen sulfide in the gas stream intoelemental sulfur and water.
 6. The desulfurization method of claim 5,further comprising depositing the elemental sulfur from the gas streamon a sulfur adsorption material within the sulfur scrubber station (52)so that an amount of the deposited elemental sulfur held by the materialas adsorbed sulfur is between about fifteen percent to about sixtypercent of a weight of the sulfur adsorption material.
 7. Thedesulfurization method of claim 5, further comprising replacing thesulfur adsorption material within the sulfur scrubber station (52) atpredetermined intervals.